Radio-wire communication system



Dec. 6, 1949 E. A. JENSEN Er AL I 2,490,061

RADIO-WIRE COMMUNICATION SYSTEM STA f/olv N0. 3

324 Amplifie mplifier 1 N V EN TORS,

Evan. CZ Jensen,

BY Kenneih J. E/zead @641; a aizfr feys Patented Dec. 6, 1949 RADIO-WIRECOMMUNICATION SYSTEM Evan A. Jensen, Westchester, and Kenneth J. Rhead,La Grange, Ill., assignors to United Air Lines, Inc., Chicago, 111., acorporation of Delaware Application July 31, 1947, Serial No. 764,994

26 Claims.

This invention relates to radio communication systems for aircraft; moreparticularly, to systems for effecting two-way radio communicationbetween aircraft, and between aircraft and ground, over distances inexcess of the normal range of transmission and reception at theoperating frequencies; and the invention has for an object the provisionof economical and reliable systems of this character.

Communication systems of the type indicated, operating on a singlechannel or assigned frequency, should, in order to achieve maximumutility, be so constructed and arranged that any ground station maycommunicate with any aircraft operating over the system, that anyaircraft may communicate with any ground station in the system overwhich it is operating, that all ground stations may communicate witheach other, that all aircraft and all ground stations may know when thesystem is in use so that interference will not be had, and that allaircraft operating over the system may hear all communications whetherfrom ground stations or other aircratf so as to be continuously advisedof operating conditions throughout the entire system. At present all ofthese requirements are being met by aeronautical communication systemsemploying long-distance high-frequency radio equipment.

With the development of VHF equipment, i. e., equipment capable ofoperating at very high frequencies, such for example as 100 megacycles,it has become desirable for various reasons to employ these very highfrequencies in aeronauti cal communication systems. Radio equipmentoperating at such very high frequencies, however, has a very limitedrange, and consequently to satisfy the above requirements it isnecessary to employ a plurality of spaced ground stations interconnectedby suitable wire circuits, such as telephone. or Teletype circuits.

Perhaps the simplest arrangement that could be employed for suchpurposes, at least from the equipment standpoint, is to have eachstation individually controlled by its individual operating staff, whowould relay messages by telephone or teletype to all of the other groundstations. With this arrangement many of the messages between ground andair would be secondhand, message transmission would be comparativelyslow, and the operating personnel would necessarily be maintained on a24-hour basis. A second arrangement, which would readily occur to thoseskilled in the art, would be to provide selective equipment enabling theoperator at any control station to select one or more transmittingstations in the system and operate the transmitters at those stations.For example, a Chicago operator desiring to communicate with an aircraftover Des Moines, could, by suitable wire circuits, select the Des Moinestransmitter and communicate directly with the desired aircraft. One ofthe principal disadvantages of a selective system of this kind is thatthe operator must know the location of the aircraft in order to selectthe proper transmitter. Furthermore, interference between messages wouldinevitably occur whenever two or more aircraft operating out of range ofeach other, and therefore unadvised as to the activities of the other,should attempt to communicate with a ground station at the same time.Having in mind the above requirements, it is a further object of thisinvention to provide a system for effecting two-way radio communicationbetween aircraft and ground which will meet all of the requirements ofmaximum utility, which requires a minimum of operating personnel, andwhich permits direct communication with all aircraft operating over thesystem either from other aircraft or from ground, and with all groundstations in the system either from other ground stations or fromaircraft operating over the system.

It is another object of this invention to provide a system of the abovecharacter wherein a message originating at any ground station isreproduced and rebroadcast at each ground station in the system.

Another object of the invention is the provision of a system of thischaracter wherein an aircraft-initiated message when received at any onestation is reproduced at all of the stations in the system but isrebroadcast only from ground stations outside of the range of theinitiating aircraft.

Still another object of this invention is the provision of a system ofthis character wherein only one receiving station operates to distributean aircraft-initiated message throughout the system for reproduction andrebroadcast even though the message is received at more than one groundstation.

A further object of the invention is to provide a system of thischaracter wherein the possibility of interference, by reason of thereception of a message at an aircraft from more than one groundtransmitter, is prevented by dividing the receiver band-pass acceptancechannel at the assigned frequency into a plurality of subchannels andenergizing adjacent transmitters in the system at closely controlleddiiferent subchannel frequencies to provide a resultant heterodyne beatnote of a frequency high enough to be readily filteredout at theaircraft receivers.

In carrying out the invention in one form, a plurality of groundstations are provided, spaced apart distances greater than the normaltransmitting and receiving range at ground level, but close enough toprovide overlapping zones of ground transmitted signals at the usualaircraft. altitudes, and close enough to permit simultaneous receptionat a plurality of ground stations of aircraft signals transmitted atsuch altitudes, and means are provided extending between the groundstations and forming voice and control energy channels, together withmeans automatically responsive to the initiation of a message at any oneof the ground stations for reproducing and broadcasting the message ateach ground station, and means automatically responsive to the receptionof an aircraft-initiated message at any one of the ground stations forreproducing the message at all of the stations and for rebroadcastingthe message only from ground stations outside of the range of themessage-initiating aircraft. In addition, timing means may be providedat each station having diiferent time settings at adjacent stationswhereby in the event of simultaneous reception of a message at more thanone station, all but one of the receivers will be disabled so that audioand control energy will be supplied to the voice and control channels ofthe system from only one of the receiving stations, the timing means inaddition being arranged to disable the transmitter at each of theStations wherein the message is being received even though the receiverat that station isv disabled.

In accordance with another aspect of the invention, the transmitters atadjacent stations are operated at different subchannel frequencieswithin the band-pass channel of the aircraft receivers, and frequencycontrol means are provided at each station for holding the transmitterclosely on the subchannel frequency, the frequency control means beingenergized, during intervals when, no message is being transmitted, by acommon standard-frequency tone transmitted to all of the ground stationsover the voice channels and adapted to be automatically removed from thevoice channels whenever a message is initiated.

For a more complete understanding of the invention, reference should nowbe had to the drawings, in which:

Figs. 1a and 1b, when placed side by side, with the right-hand side ofFig. 1a adjacent the lefthand side of Fig. 1b, illustratediagrammatically a communication system embodying the present invention;

Fig. 2 is an explanatory diagram illustrating the manner in which aplurality of ground stations may be spaced apart and interconnected toprovide overlapping signal zones at various aircraft altitudes;

Fig. 3 is a circuit diagram illustrating the frequency control meansemployed at each of the ground stations of the system illustrated inFigs. 1a and lb;

Figs. and 5 are explanatory diagrams illustrating certain operatingconditions that may be encountered with systems embodying the presentinvention; and

Fig. 6 is a circuit diagram showing automatic. control means forinsuring the achievement of the operating condition shown in Fig. 5 andpreventing the disadvantageous condition illustrated in Fig. 4.

Referring now to the drawings, the invention is shown as embodied in acommunication system comprising two ground stations, respectivelyidentified as Station No. 1 (Fig. 1a) and Station No. 2 (Fig. lb). Inorder to simplify the drawing, single lines have been employedthroughout to illustrate the various circuits, but it will be understoodthat each circuit includes the usual return circuits and groundconnections. For convenience the single lines employed in the drawingwill be referred to throughout as conductors, but it will be understoodthat each single line may constitute more than one conductor and suchadditional circuit elements as may be necessary in each instance inaccordance with standard practice. Stations No. 1 and No. 2, as will bemore fully explained hereinafter, are connected by suitablelong-distance telephone lines, exemplified' in the drawings by a voicechannel ll] capable of high-speed transmission of audio-irequencies, andby a control channel l i which may constitute any type of circuitcapable of transmitting control energy; and it will be understood thatthe two stations illustrated are merely exemplary of a larger number ofidentical stations all of which are connected to the long-distance voiceand control channels. For example, in Fig. 2 a system embodying theinvention is diagrammatically indicated as comprising ground stationsl2, l3, it, and i5 connected by suitable long-distanc telephone linesll. While a separate control channel is employed in the embodiment ofthe invention shown, it will be understood that the same circuits thatcomprise the voice channel may be employed for transmitting suitablecontrol signals, thus eliminating the ne cessity for a physicallyseparate control channel.

Referring now specifically to Fig. la, Station No. 1 of the system isshown as comprising an operating site 58 and a transmitting andreceiving site 59, which are in the usual case located at some distancefrom each other and are therefore connected by local voice and controlenergy channels, respectively represented in Fig. lit by the conductors2i], 2!, and 22, 23.. It may be desirable in some installations toconnect more than one operating site to a single transmitting andreceiving site, and this may readily be accomplished through localtelephone service circuits, as indicated by appropriate legends on thedrawing.

As shown, the operating site I8 comprises a source of control energy 23, a sound pickup device, which is illustrated as comprising amicrophone 25 connected to a suitable constant level amplifier 26, asound reproducing device, which is illustrated as comprising a headphoneset 21 connected to a suitable amplifier 28, and a pair of automaticswitching devices or relays 29 and 30 for selectively connecting thesound pickupand sound reproducing devices to the voice channels andconnecting the source of control energy to the control channels, as willbe more fully explained hereinafter.

As shown, the output circuit of the microphone amplifier 25 and theinput circuit of the headset amplifier 28 are interconnected in theusual man ner through a manually operable press-to-talk' switch 3! and aconventional hybrid coil 32, which latter is employed for anti-side-tonepurposes and is adapted to be connected to the voice channels.

through the contactsof: the relays 29. and 3a. In

addition, a suitable manually operable switch 33 may be provided, asshown, for isolating the station from the long-distance voice andcontrol channels if desired. The various devices going to make up theoperating site of the ground station and the interrelation therebetweenwill be more fully explained hereinafter in the description of theoperation of the system.

The transmitting and receiving site l9 includes a source of controlenergy 34, a conventional VHF receiver 35, a time control switch ortiming device 36, a conventional VHF transmitter 31, a frequencycontrolling device 38 for supplying a controlled frequency to thetransmitter, and a frequency generator 39 for producing astandardfrequency tone the purpose of which will be more fully set forthhereinafter. As previously indicated, the VHF receiver 35 is ofconventional construction and includes an audio-output circuit,represented by the terminal 40, and a carrier control circuit,represented by the terminal 4|, the carrier control circuit, as will beunderstood by those skilled in the art, being energized only when acarrier of the proper frequency is being received.

The time control switch 36 may be of any desired type available on themarket, and is shown for purposes of illustration as comprising astepping switch having three banks of contacts 42, 43, and 44 adapted tobe sequentially engaged by movable contact arms 45, 46, and 41,respectively, when the switch is energized with pulses obtained from analternating current source through a half-wave rectifier 48. Timecontrol switches of this general character are well known in the art,and accordingly it is not believed necessary to illustrate the actualoperating mechanism em ployed other than to indicate by the broken lines49 that the three contact arms 45, 46, and 41 are connected together forsimultaneous operation. It will be understood that other types of timingdevices or circuits may be employed, if desired, in place of the timecontrol switch 36.

The VHF transmitter 31 is of the type including an audio-input circuit58, a control frequency input circuit 5i adapted to be energized fromthe usual crystal-controlled oscillator to produce the desired carrierfrequency, and a carrier control circuit represented by the terminals52, which circuit when opened and closed is eifective to turn thetransmitter on and off, i. e., to initiate and terminate thetransmission of a message. In addition, the transmitting and receivingsite 59 includes a number of automatic switching devices or relays 53,54, 55, 56, and 51 for controlling the receiver and transmitter and thevarious devices associated therewith. Finally, the site 19 includes asuitable hybrid coil 58 the purpose and operation of which will beexplained hereinafter.

Referring now to Fig. 1b, the equipment included in both the operatingsite and the transmitting and receiving site of Station No. 2 is, withminor exceptions, identical with the equipment at Station No. 1.Accordingly the same reference numerals with the suffix a added theretowill be used to identify corresponding equipment and components. Theexceptions to the identity are that Station No. 2 does not include astandard frequency generator such as the generator 39 at Station No. 1,and consequently the relay 51a at Station No. 2 requires only one pairof contacts instead of the two pairs of contacts required by relay 51 atStation No. 1.

The general nature and the arrangement of some or theprincipal'components of the system having now been described, it isthought that anunderstanding of the invention may best be had from adetailed description of the operation of the system during thetransmission and reception of both ground-initiated andaircraft-initiatedsignals. In Figs. 1a and 1b of the drawing the systemis illustrated with the various components in the respective conditionscorresponding to an interval during which no signal is being eithertransmitted or received. During such an interval the frequency generator39, for supplying a standard tone to the frequency controlling equipment at the various ground stations, is connected through the conductor59, the contacts 60 of the relay 51, conductors BI and 62, the contacts83 of the relay 51, and a conductor 64 to the fre-' quency controllingequipment 38 which, as heretofore indicated, functions to supply aclosely controlled frequency to the transmitter 31 in a manner whichwill be more fully described hereinafter. Likewise the frequencygenerator 39 supplies a standard tone to the frequency controllingequipment 38a at Station No. 2 by way of a circuit which may be tracedthrough the conductor 59, the contacts 60, and the conductors 6| and 65to the local voice channel 2| at the transmitting and receiving site l9,thence through the voice channel 28 at the operating site [8, and by wayof the upper pair of contacts on the switch 33 to the long-distancevoice channel 18. At Station No. 2 the circuit extends from the voicechannel in through the upper contacts on the switch 33a, and thencethrough the local voice channel 28a and am, a conductor 65a, contacts63a on the relay 51a, and the conductor 64a to the frequency controllingequipment 38a. Thus during the time intervals when the system is not inuse for the transmission or reception of messages, a standard toneproduced by the frequency generator 39 will be supplied to the frequencycontrolling equipment at the various stations.

Assuming that the operator at the operating site l8 at Station No. 1desires to transmit a message, the first step is to operate thepress-totalk switch 3|, thereby to connect the amplifier 26 of themicrophone 25 to the middle tap of the hybrid coil 32. The press-to-talkswitch 3| includes additional contacts 66 and 61 which, upon closure,effect energization of the relays 29 and 30, respectively, theseenergizing circuits extending from the source of control energy 24through a conductor 58, the contacts 61, and conductors 69 and 19 to theoperating winding of the relay 29, and from the source of control energy24 through a conductor 1|, the contacts 66 and conductors 12 and 13 tothe operating winding of the relay 30. Operation of the relay 38effects, through closure of its contacts 14, connection of themicrophone 25 to both the long-distance voice channel It and local voicechannel 20, the circuit being traceable from the right-hand terminal ofthe hybrid coil 32 through conductors 15 and 16, the contacts 14, and byway of conductors 11 and 18 to the common terminal 19 to which both ofthe voice channels are connected. Simultaneously closure of the contactson the relay 30 eifects connection of the source of control energy 24 tothe long-distance control channel I I, through a circuit which extendsfrom the source 24 through conductors 8| and 82, the

contacts 88, and a conductor 83 to the control channel ll.

Similarly, operation of the relay 29 is effective, through closure ofits contacts 84, to complete a circuit which includes the conductors 85and 86,

and which is in' parallel relation to the circuit including the contactsi l of the relay 30 through which the hybrid coil 32 is connected toboth the long-distance and local voice channels, these parallel circuitsinsuring connection of the micro-- phone 25 and the headset 2'! to thevoice channels upon operation of either of the relays 29 or 36. Finally,operation of the relay 29 is eifective, through closure of its contacts8'5, to connect the source of control energy 2t to the local controlchannel 22, 23, the circuit being traceable from the source 2 throughconductor 85, a conductor 88, the contacts 8?, and the conductor 89 tothe control channel 22.

Connection of the microphone 25 to both the long-distance and localvoice channels, and connection of the source of control energy 24 toboth the local and long-distance control channels are effective, as willnow be described, first to disconnect the frequency generator 39 and allof the frequency controlling equipment from the voice channels; second,to connect the microphone 25 to the headset Zfic; third, to connect themicrophone to the audio-input of all of the transmitters; and fourth, toturn on the transmitters so that the message originating at themicrophone 25 will be broadcast from all of the ground stations in thesystem.

Referring first to Station No. 1, it will be seen that as soon as thelocal control channel 23 is energized, the relay will be operated, thecoil of the relay being connected to the control channel 23 through aconductor 9%}. Thus the contacts 69 are opened to disconnect thefrequency generator 39, and the contacts 63 are opened to disconnect thefrequency control equipment 38 from the voice channel 2 i.Simultaneously with operation of the relay 5?, the relay 53 is operated,the energizing circuit for relay 53 being traceable from the controlchannel 23 through a conductor 9!, the upper portion of the hybrid coil58, and a conductor 92 to the operating coil of the relay 53. Uponoperation of the relay 53, the contacts 93 thereof close to connect theaudio-input of the transm' ter to the voice channel 2i through aconductor at, the contacts 93, and a conductor 95. Likewise, closure ofthe contacts 96 on the relay 53 completes a circuit between the carriercontrol terminals '52 of the transmitter; which circuit may be tracedfrom the upper one of the terminals through a conductor 91, a pair ofnormally closed contacts 98 on the relay 5B, the conductor 89, thecontacts 96 on the relay 53, and a conductor We to the lower one of theterminals 52. Closure of this circuit turns on the transmitter so thatthe message originating at the microphone 25 will be broadcast'by thetransmitter 37.

Referring now to No. 2, energization of the long-distance cont 1 channel11, as hereinbefore explain-ed, effective to cause operation of therelay 255a, the energizing circuit extending from the control channel llthrough a conductor lfila, a pair of normally closed contacts [02a onthe relay and conductors liiSa and 18a to the operating winding of therelay 29a Operation of the relay 29a is effective, through closure ofcontacts t le, to connect the headset to the voice channel ID by way ofa circuit which extends from the voice channel ten inal led throughconductors 78a and contacts tea, and conductors 85a and its to the endterminal of the hybrid coil 32a to which the headset amplifier 28a isconnected, LilseWise, relay 29ais effective,

through closure of its contacts 87a, to energize the local controlchannel 22a, 23a at Station No. 2 from the local source of controlenergy 24a, this circuit being traceable from the source of energy 24athrough the conductors 81a and 88a, the contacts 87a, and the conductor89a to the control channel 22a. Energization of the control channel 23aat the transmitting and receiving site l9a of Station No. 2 effectsoperation of the relay 51a to disconnect the frequency control equipment38a from the local voice channel, the operating winding of the relay5111 being connected through conductor 99a to the control channel 23a.Likewise the relay 53a will be energized through a circuit extendingfrom the control channel 23a, a conductor 99a, the upper half of thehybrid coil 58a and conductor 92a which leads to the operating windingof the relay 53a. As previously described in connection with Station No.1, operation of the relay 53a is effective, through the closure of itscontacts 93a and 95a, to connect the audio-input of the transmitter tothe voice channel and to turn on the transmitter, the circuits beingidentical with those previously traced in the description of Station No.1.

From the above description it will be apparent that whenever thepress-totalk switch is operated at Station No. l, the frequencygenerator 39 will be automatically disconnected from the voice channels,and a message originating at microphone 25 will be broadcast from eachof the ground stations. Upon release of the press-totalk switch, therelays 29 and 38 at Sta-tion No. 1 will immediately be de-energized todisconnect both the microphone 25 and the control energy source 2t fromthe local and long-distance voice and control channels, whereupon therelays 53 and 51 will be dc-energized to turn ofi the transmitter andreconnect the frequency controlling equipment 33 and the frequencygenerator 39 to the voice channels. Likewise the relay 29a at StationNo. 2 will be de-energized to disconnect the source of control energy24a from the local control channel 22a, 23a, and the relays 53a and 57awill be lie-energized to turn off the transmitter Bio and to reconnectthe frequency channels.

The operation of the system to effect broadcasting from all of thetransmitters of a message originating at Station No. 2 upon operation ofthe press-to-talk switch a will be apparent from the operationheretofore described, the circuits being identical and interchangeableat both stations. Accordingly the various circuits involved for insurinbroadcasting of the message originating at Station No. 2 will not bedescribed in detail. Thus, whenever a message is initiated at any one ofthe ground stations, that message will be reproduced at each of theother ground stations and will be transmitted from all the transmittersin the system so as to reach any aircraft operating over the system eventhough the aircraft is completely otuside of the range of the initiatingstation.

It will now be assumed that with the equipment in the normal positionillustrated in the drawing, an aircraft-initiated message is received atthe receiver 35 of Station No, 1. As soon as this message reaches thereceiver 35, the carrier control equipment of the receiver will supplyenergy from the terminal 4| through conductors I04 and ms, the normallyclosed contacts 106 of therelay 53, and by way of conductors l0]controlling equipment 38a to the voice and I08 to the energizing windingof the relay 55, and the consequent operation of the relay 55 iseffective, through closure of its contacts I69, to supply energy fromthe control energy source 34 to the time control switch 36 throughconductors H9, III, H2, and N3, the contacts I69, and a conductor II4 toone terminal IE5 of the time control switch. The source of controlenergy 34, as well as all of the other heretofore referred-to controlenergy sources, is preferably a standard Gil-cycle source of alternatingcurrent, but it will be understood that other frequencies may be used,or if a different type of time control switch or timing device isprovided, direct current may be employed.

In the arrangement shown, it will be assumed that the source of energyis GO-cycle alternating current, and upon connection of the terminal I5to the source 34, the time control switch 36 will begin its steppingoperation clue to the fact that pulses of energy will be supplied to theoperating winding I I 5 of the switch through a circuit which extendsfrom the terminal II5 through conductors III and I It, the four contactsat the left of the contact bank 42, the contact arm 45, and therectifier 48. The operating mechanism of the time control switch is suchthat each pulse of energy will cause the switch to move one step, thatis, from one contact to the next at each pulse, and consequently theswitch 36 will operate through four steps until the contact arm 45engages the contact IE9 of the bank 42, thus interrupting the previouslytraced energizing circuit. Simultaneously with the engagement of thecontact arm 45 and the contact I I9, the contact arm 46 on the bank 43will engage contact I29 of this bank to complete an energizin circuitfor the relay 54, which energizing circuit may be traced from thepreviously energized conductor I I1 through a conductor Hi, the contactI20, the contact arm 45, the conductor I22, terminal I23, conductor I24,the normally closed contacts I25 on the relay 53 and by way of aconductor I26 to the operating winding of the relay 54.

Operation of the relay 54 is effective through closure of its contactsI27! and I28 to connect the source of control energy 34 to the controlchannel 23 and to connect the audio-output of the receiver 35 to thelocal voice channel 2 I. The

control circuit thus completed extends from the source of energy 35through the conductors III] and I39, the contacts I2'I, a conductor I29,the upper portion of the hybrid coil 56, and conductor 9! to the controlchannel 23. It will be observed that the conductor I29 in this circuitis connected to the center terminal of the hybrid coil 58, andaccordingly control energy, while passing through the upper portion ofthe hybrid coil to the conductor 9!, will not pass through the hybridcoil to the conductor e2, whereby the relay 53 remains de-energized andconsequently" the transmitter 3? at Station No. 1 will not be turned on.Energization of the control channel 23, however, is effective, throughthe conductor 99, immediately to energize the relay 5? so as todisconnect the frequency generator 39 and the frequency controlequipment 38 from the voice channel 2!. The circuit for the audio-outputof the receiver may be traced from the terminal 46 of the receiverthrough a conductor I3I, the contacts 529 on the relay 5%, and by way ofa conductor 32 to the local voice channel 2i.

Since the local voice channel 25, 2I is connected, as herein described,to the long-distance voice channel I9, theaudio-output of the receiver35 will thus be suppliedto the long-distance voice channel as well asthe local voice channel at Station No. 1. Energizing of the localcontrol energ channel 23, 22 is effective to cause operation of therelay 39 at the operating site I8 of Station No. 1, this circuit beingtraceable from the control channel 22 through a conductor I33, a pair ofnormally closed contacts I34 on the relay 29, and a conductor I35 to theoperating winding of the relay 36. Operation of the relay 30 iseifective, through closure of its contacts 14', to connect the headset2I at Station No. l to the voice channels whereby the message receivedat the site I9 is reproduced at the operating site I8 of Station No. 1,this latter circuit extending from the terminal I9 of the voice channel29 through the conductors I8 and 11, the contact I4 on the relay 30, andthe conductors I6 and I5 to the hybrid coil 32 to which the input of theamplifier 28 is connected.

Likewise operation of the relay 39 is effective, through closure of itscontacts 89, to supply control energy to the long-distance controlchannel II from the control energy source 24 through the conductors BIand 82, the contacts 89, and the conductor 83 which is connected to thecontrol channel II. Energization of the control channel I I from thesource-24 is effective, as previously described in connection with themessage originating at the microphone 25, to efiect operation of therelay 29a at Station No. 2, and through closure. of the contacts 84 and81a on the relay 29a to connect the headset 21a to the voice channel andto connect the source of control energy 24a to the local control channel22a. Thus the message received at Station No. 1 will be reproduced atthe headset 21a at Station No. 2, and

at Station No. 1 will be reproduced at all of the ground stations andwill be rebroadcast at stations other than Station No. 1 so as to beavailable to other aircraft operating over the system outside of thetransmitting range of the initiating aircraft.

It will be recalled that initial energization of the relay from thecarrier control circuit of the receiver 35 was accomplished through acircuit including the normally closed contacts I96 on the relay 53, andimmediately upon operation of the relay 55 a self-holding circuit iscompleted which extends from the carrier control terminal 4|, throughthe conductors I94 and I36, the contacts I31 on the relay 55 and by wayof conductors I38 and I08 to the energizing winding of the relay. Thusthe relay 55 will remain energized to maintain the previously describedcircuits so long as the message is being received by the receiver 35.Immediately upon termination of the message, however, the relay 55 willbe de-energized, whereupon the contacts Iil9 open to interrupt theenergizing circuit for the winding of the relay 54 and cause opening ofthe contacts I21 and I28 to disconnect the audio-output circuit of, thereceiver and the source of energy 34 from the local voice and controlchannels 2I and 23.

De-energization of the control channel 23 effects de-energization of therelay to reconnect the frequency generator 39 and thefrequencycontrolling equipment 38 to the voice channel 2 I, as previouslydescribed. In addition, de-energization of the control channel 23effects deenergization of the relay 30 whereupon the previously tracedcircuits are interrupted at the contacts I4 and 80 to disconnect theheadset 2? from the voice channels and to disconnect the source ofcontrol energy 24' from the long-distance control channel H. Aspreviously described, deenergization of the control channel II de-enervgizes the relay 29a at Station No. 2, thereby disconnecting the headset21a from the voice channels and disconnecting the control energy source24a from the local control channel 22a at Station No. 2, whereupon therelay 53a Station No. 2 is de-energized to turn off the transmitter 31a,and the relay 51a is de-energized to re-connect the frequency controlequipment 38a to the voice channels for energization by the standardtone produced by the frequency generator 39 at Station No. 1.

Returning now to Station No. 1, de-energization of the relay 55 not onlyinterrupts the energizing circuit for the relay 54 as previouslyexplained, but through closure of the contacts I39 on the relay 55establishes an energizing circuit for returning the time control switch36 to its starting position. This energizing circuit extends from thecontrol energy source 34 through the conductors Ilil, III, and I40, thecontacts I39, and a conductor I4I to the terminal I42 of the timecontrol switch. From this terminal the circuit extends through aconductor I43 to the normally de-energized contact I I9 on the contactbank 42 of the time control switch, thus completing a circuit from thecontact II9 through the arm 45 and the rectifier 48 to the operatingwinding of the time control switch. As soon as this circuit iscompleted, the arms 45, 46, and 41 of the time control switch will moveforward one step, thus immediately interrupting the .just tracedenergizing circuit, but upon contact of the arm 45 with the adjacentcontact I44 on the bank 42, another energizing circuit will be completedwhich extends from the source 34 through the conductors IIO, III, H2,and I45 to the terminal I46 of the time control switch, and from thisterminal through conductors I41 and I 48, the contact I44, the movablearm 45, and the rectifier 48 to the operating winding IIS. Thus the timecontrol switch will continue its stepping operation until the contactarms 45,.46, and 41 return to the position shown in the drawing, thelast energizing circuit for the time control switch being interrupted assoon as the arm engages the dead contact I49 on the bank 42. Thus itwill be seen that immediately upon termination of the aircraft-initiatedmessage, all of the equipment at each of the ground stations willautomatically return to its normal position, and the standard frequencytone produced by the generator 39 will again be applied to the voicechannels to energize the frequency control equipment at each of thestations.

In the above-described operation it was assumed that theaircraft-initiated message was received only at Station No. 1. Undersuch conditions the message will be reproduced at all of the groundstations but will be rebroadcast only at stations other than the onewhere it is being received. It will be understood, however, thatconditions will arise in the operation of systems embodying thisinvention wherein an. aircraftinitiated message may be received at morethan one ground station. In Fig. 2 a system is shown which includes fourground stations, I2, I3, I4, and I5, the transmitting range of thestations being respectively indicated by the curves I2, I3, I4, and I5.Although the patterns may be somewhat different, these curves may alsobe taken as approximations of the receiving range of the stations, or inother words, transmitting range of aircraft operating over the system.Thus an aircraft operating at the point designated X in Fig. 2 would,when transmitting, be able to communicate directly only with station I3,and the message would be forwarded from station I3 to the other stationsI2, I4, and I5 in the manner just described, station I 3 correspondingto Station No. 1 in this previous description, and each of stations I2,I4, and I5 corresponding to Station No. 2.

On the other hand, if the transmitting aircraft is operating at thepoint Y in Fig. 2, the transmitted message would be received not only atstation I3, but also at stations I2 and I4. Consequently, unless specialmeans were provided, each of these stations would, upon receipt of themessage, attempt to supply audio and control energy to the voice andcontrol energy channels, which might result in objectionableinterference. In accordance with the embodiment of the invention shown,the possibility of such interference is prevented, and only one of thestations receiving the message supplies the control energy and the audioto the voice and control energy channels for transmission throughout therest of the system. This is accomplished by assigning difierent timesettings to the time control switches at all of the stations that liewithin the range of the transmitting aircraft. Thus in Figs. 1a and 1bit will be observed that the time control switch 36 operates throughfour steps before the contact arm engages the normally dead contact H9in bank 42, while in the time control switch 36a the arm 45a need onlymove through three steps before engaging the corresponding contact II9a. Thus when GO-cycle control energy is employed, the time controlswitch 36a will have a setting of of a second, while the time controlswitch 36 has a setting of In the system of Fig. 2, for example, stationI2 might be assigned a time setting of /60 of a second; station I3 asetting of A30; station I4 a setting of /60; and station I5 would thenhave a setting of /60, the same as station I'2, since these two stationsare sufficiently spaced so that they would not, at the frequenciesemployed, ever receive the same signal from an aircraft. Referring againto Figs. 1a and b, it will be assumed that an aircraft-transmittedmessage is received at both Station No. 1 and Station No. 2. Aspreviously explained, the receivers 35 and 35a thereupon supply controlenergy from the carrier control circuits to energize the relays and 55a,whereupon the time switches 36 and 35a begin operating. As soon as thetime switch Sta completes its three-step cycle, the relay 54a will beenergized through the contact bank 43a and the contact arm 46a, aspreviously described in connection with the time switch 36, whereuponcontrol energy and the audio-output of the receiver 35a will be suppliedto the voice channels and the control channels throughout the system.This will result in connection of all of the headsets 21 and 21a to thevoice channels, in disconnection of the frequency control equipment 38and 38a a d e S d tone. nerator 39 from the voice channels, and inoperation of the relay 53 at Station No. l to connect the audio-input ofthe transmitter 3! to the voice channels and to turn on the transmitter,all as previously explained in connection with a condition wherein themessage was received only at Station No. 1, the operation now beingreversed because Station No. 2 has the lower time setting.

Inasmuch as Station No. l is within the transmitting range of theaircraft, it is undesirable to have this station re-broadcast themessage, and consequently means are provided for disabling thetransmitter at each of the stations wherein the message is beingreceived even though only one of these stations is supplying the controlThis is accomplished by energy in the audio. means of the time switch 35which completes its cycle /60 of a second after the completion of thecycle of the switch 35a. When the contact arm 46 in the bank 43 of timeswitch 35 engages the contact I25, energization of the relay 54 atStation No. 1 cannot occur because the relay 53 has now operated to openthe contacts I25 in the energizing circuit of the relay 54. At the sametime, however, the contact arm 41 engages contact I55 on contact bank Mand completes an energizing circuit for relay 55. This energizingcircuit extends from the control energy source 34 through conductors II0, I I I, I I2, and I45, terminal I45, conductors I41 and I5I, contactI55, K

contact arm 51, conductor I52, terminal I53, conductors I54 and 55,contacts I55 on relay 53, which contacts are now closed due to the factthat relay 53 has been operated by the control energy supplied fromStation No. 2, and by 9 way of conductors I51 and I58 to the operatingwinding of the relay 55. Operation of the relay 55 is effective to turnoff the transmitter 3'! by opening contacts 98, which contacts form apart of the previously closed on-and-off circuit of the transmitter.lhus even though the relay 53 has operated so as to prevent audio fromthe receiver 35 being supplied to the voice channels through the relay54, the transmitter 3'! is rendered inoperative by operation of therelay 56, which means that the message received at Station No. 2 will berebroadcast only at stations outside the receiving range and not by anystation wherein the receiver is directly receiving the message. Closureof the relay 55 is effective to establish a self-holding circuit whichextends from the junction of the conductors I55 and I55 in thepreviously traced energizing circuit, through a conductor I59, thecontacts 55 on the relay 55, and conductors NH and E58 to the operatingwinding of relay 56. Thus the relay 56 will remain energized so long asthe associated receiver is receiving the message, and will bedeenergized only when the relay 55 operates as previously described tocause the time switch to return to its normal position, the contact armll then moving off the contact I50 to interrupt the energizing circuitof the relay 55.

As will be apparent to those skilled in the art, whenever one of thetransmitters, for example the transmitter 5'? at Station No. is turnedon either by initiation of a message at one of the ground stations or byreception of an aircraftinitiated message at some station outside of thereceiving range of Station No. l, the receiver 35 at Station No. i willreceive the transmission from its associated transmitter and would, inthe absence of proper preventative circuits, immediately function toenergize the time switch 36 and disable the transmitter through therelay 55, whereupon reception by the receiver 35 would ceiver 35, theenergizing circuit for the relay 55' is opened and the receiver 35 isrendered inca pable of energizing the relay 55 whereby the time switch35 and the relay 55 remain de-energized. If, however, the relay 53 doesnot operate to turn on the transmitter until after the relay 55 has beenoperated by the reception of a message, the contacts I55 are ineffectiveto de-energize the relay 55 due to the previously traced self-holdingcircuit which includes the contacts I31. Accordingly, the receivercontrol circuits, which are normally operable to disable the transmitterwhenever a message is being received, are not effective when thereceived message is being transmitted at the same station.

As previously explained, adjacent ground stations in systems embodyingthis invention although located out of range of each other at groundlevel, are close enough to provide, as shown in Fig. 2, overlappingzones wherein an aircraft may receive messages from two or more groundstations, which would of course result in an interfering audibleheterodyne unless all of the ground station transmitters are heldexactly on A frequency. Such exact frequency control could beaccomplished by energizing the local frequency control equipment at eachstation continuously from a common standard frequency tone supplied overa separate voice channel capable of transmitting such a tone. However,the stand-by toll charges on such an additional voice channel result inundesirably high fixed operating charges, and in accordance with thepresent invention the necessity for an additional voice channel isobviated by employing the same voice channel for transmission of thestandard frequency tone as is employed for transmitting theaudio-frequency voice currents, the standard.

sufficiently high in frequency to permit ready filtering at thereceiver. For example, if the aircraft receivers are designed, inaccordance with standard design practice, to provide a normal band passof 50 kilocycles, and proper allowances are made for the normaltolerance of the receiver crystals and the standard frequency tone, aswell as for certain other known variable are tors, a usable channelapproximately 25 kilocycles in width will remain, which may be subdivided into four subchannels having a frequency separation ofapproximately 8 kilocycles, and these subchannel frequencies areassigned to adjacent ground stations.

If the stations in a system embodying the present invention are locatedgeographically in approximately a straight line with sufficient distanceseparation between stations so that alternate stations would provide nooverlapping signals zones, only two subchannel frequencies would berequired. However, if as shown in Fig. 2, the distance between suchstations is such as to permit overlapping between signals transmittedfrom stations I2 and I4, for example, three subchannel frequencies wouldbe required, as indicated in Fig. 2. If the system should includestations arranged so as to provide a crossover, four subchannelfrequencies would be required, and although more than four subchannelscould be achieved by accepting a lower heterodyne beat note at thereceivers, it has been determined that four subchannel frequenciesshould take care of substantially all required network arrangements.With the spaced subchannels assigned to the various stations asindicated above, exact frequency control is not required, and variationsof plus or minus 500 cycles or even more may be tolerated. This isachieved in systems embodying the present invention either by the use ofclose tolerance quartz crystals at each station for holding therespective transmitters on the assigned subchannel frequencies withinthe permissible variation, or by providing the frequency controllingequipment 38 and 38a at the various stations and employing the standardfrequency tone to maintain this frequency controlling equipment exactlyon frequency except during the short intervals of message transmission.In the event that close tolerance quartz crystals of sufficient accuracyare available and are employed in the system, the standard frequencygenerator 39, the frequency control equipment 38, and the control relaysand circuits therefor, now to be described, may be omitted from thesystem.

Referring to Fig. 3 of the drawing, the frequency controlling equipment38 for supplying control frequency to the transmitter 31 is showndiagrammatically as including a local oscillator I62 for producing thefrequency necessary to insure operation of the transmitter at theassigned subchannel frequency. The oscillator I62, which is of standardconstruction, is shown as including a crystal I63 and a variable paddingcondenser I64, the output of the oscillator being connected to thecontrol frequency terminal 5| (Fig. 1a) of the transmitter 31 by aconductor I65. The standard frequency tone produced by the frequencygenerator 39 is supplied, as previously explained, to the frequencycontrol equipment 38 through the conductor 64, where it is multiplied ina suitable frequency multiplier I66 to a frequency which differs, by aselected audiofrequency difference, from the frequency which theoscillator I62 is intended to hold. The multiplied frequency is thenfed, as indicated in Fig. 3, to a suitable detector or mixer I61, whereit is heterodyned with the output of the oscillator I62 to provide sumand difference beat-note frequencies, the oscillator being connected tothe detector I61, as indicated, by the conductor I68. The differencefrequency beat note appearing at the output of the detector I61 isselected by a suitable audio-frequency band-pass filter I69 and isamplified in a suitable amplifier I before being fed to the inputterminal I1I of a discriminator I12.

The discriminator I12 is constructed to operate with a crossoverfrequency equal to the selected difference between the multipliedstandard frequency tone and the frequency on which the oscillator is tobe held, and comprises a pair of tuned circuits I13 and I14, respectiveltuned to frequencies above and below the crossover frequency, the tunedcircuits being connected through oppositely disposed rectifiers I15 andI16 to the opposite ends of a potentiometer I11 which is connectedacross a pair of condensers I18 having a mid-point ground connectionI19. The arrangement of the various components of the discriminator I12is such that the potential at the point I on the potentiometer I11 willvary between positive and negative values, depending upon the variationof the difference frequency supplied to the discriminator input from thecrossover frequency of the discriminator.

Thus if the difference frequency applied to the input of thediscriminator is exactly equal to the crossover frequency, zeropotential will appear at the output terminal I8I, while a variation ofthe difference frequency above or below the crossover frequency willcause positive or negative potentials, as the case may be, to appear atthe output. As shown, the output of the discriminator serves to energizean electric motor I82 having a pair of field windings I83 and i8 3connected between the output terminal I8! and a ground connection I85.The rotor of the motor I82 comprises a permanent bar magnet I86 which ismounted on a rotatable shaft I81 that is connected, as indicated by thebroken line I88, to the padding condenser I64 of the oscillator I62.

Th direction of rotation of the rotor I86 and the shaft I81 is dependentupon the direction of current flow in the field windings I83 and I64,and suitable stops I89 are provided for limiting angular movement orrotation of the shaft. It will thus be seen that the discriminator I12acts through the motor I82 to adjust the crystal oscillator I62 so thatthe difference frequency obtained by heterodyning the oscillator outputwith the multiplied standard frequency tone is maintained equal to thecrossover frequency of the discriminator. Thus the frequency supplied tothe transmitter 31 by the oscillator I62 is held at the desired value,and when the transmitter is turned on, the carrier frequency will beexactly on the assigned subchannel.

The accuracy with which the frequency control will be held dependsalmost entirely upon the accuracy with which the crossover frequency ofthe discriminator may be set and held, and the accuracy of thediscriminator calibration may be checked by comparison with the standardfrequency tone. For this purpose a suitable frequency meter I90 isprovided, connected, as shown, through a suitable push-button switchI9I, the frequency meter I90 being of the type to indicate the deviationof the difference frequency selected by the band-pass filter from thestandard frequency tone. As previously indi cated, the standardfrequency tone is continuously supplied to the frequency controlequipment 38, except during intervals of message transmission, and ithas been found that by closely controlling the crystal oscillator I62through the discriminator, as described above, the transmittingfrequency during the short intervals of message transmission will beheld within proper limits.

In Fig. 4 there is shown diagrammatically a condition of operation whichmay, under certain '17 bircumstances, occur in the operation of acommunication system embodying the present invention as thus 'fardescribed. The system here shown comprises five ground stations I92,I93, I94, I95, and I96 interconnected by suitable long-distancetelephone lines I91, which will be understood as constituting the voiceand control channels I0 and II of Figs. 1a and 11). Each of the groundstations comprises a receiver I98 and a transmitter I99, the suflixes a,b, c, and at being employed to distinguish the receivers andtransmitters at the various stations. Each of the ground stationsincludes the various components and control means heretofore describedin connection with Station No. 1 and Station No. 2 of Figs. 1a and 1bfor selectively rendering the receiver and the transmitter effective orinefiective in accordance with the operating condition of the system. InFig. 4, however, the condition of the receivers and transmitters isrepresented diagrammatically, for purposes of simplicity, by theposition of a multiple position switch 209, which is shown as beingadapted selectively to connect the receiver or the transmitter to asuitable antenna, it being understood that the switch 200 is shownmerely for diagrammatic purposes, the receivers and transmitters beingat all times connected to the antennas, as shown in Figs. 1a and 1b, andbeing rendered effective or ineffective by proper control of thereceiver output and transmitter input circuits.

It will now be assumed that an aircraft operating over the system asshown in Fig. 4, is transmitting a message and that, as indicated by thearrows 202, the aircraft-initiated message is being received at stationsI92, I93, and I94. If station I94 has the lowest time setting on itstime control switch 36, the receiver I98b will, as previously explained,deliver the audio and control energy to the voice and control channelsof the system. As previously described, and as indicated in Fig. 4 bythe neutral position of the switches 200 and 200a, both the receiversand the transmitters will be rendered ineffective at stations I92 andI93, which stations are within the transmitting range of the aircraftand have higher time settings. Likewise, as indicated by the positionsof switches 2000 and 200d, the transmitters I990 and I99d will be turnedon at stations I95 and I96 to rebroadcast, as indicated by the arrows203 and 204, the message received at station I94, stations I95 and I96being outside of the transmitting range of the aircraft.

Although as previously explained, the ground stations in systemsembodying the invention are spaced apart distances in excess of thenormal transmitting and receiving range at ground level, unusual terrainor unusual atmospheric conditions may so increase this normal range, asindicated by the extension arrows 205 in Fig. 4, as to cause therebroadcast message from station I95 to reach the receiver I982) atstation I94. Since this is the receiver which is supplying the audio andcontrol energy to the system, a closed loop, as indicated by the loop206 in Fig. 4, would be formed between the stations I94 and I95 whichmight cause an objectionable audio sing, and which under certainconditions might look up the control circuits at station I94 after theaircraft 20I has completed its transmission.

' Although this abnormal and objectionable condition may only occurunder unusual conditions or at particular points in the system, theinvention contemplates as a further aspect thereof the provision ofmeans at the ground stations for eliminating or reducing to a minimumthe possibility of such improper operation. In Fig. 5 a system embodyingthe invention and including such additional means is shown as comprisingground stations 201, 208, 209, and 2I0 connected by suitable telephonelines 2| I. Station 201, as shown diagrammatically, includes a pair ofunidirectional antennas 2I2 and 2I3 arranged to face in oppositedirections, a pair of receivers 2I4 and 2I5, and a transmitter 2I6. Itwill be understood that station 201 includes the various components andcontrol means illustrated and described as composing Station No. 1 andStation No. 2 of Figs. 1a and 1b, and as in Fig. 4, multiple positionswitches 200 are employed to indicate diagrammatically the effective orineffective condition of the transmitters and receivers as determined bythe control apparatus. In actuality, the receivers are permanentlyconnected to their respective unidirectional antennas, and thetransmitter is permanently connected either to a nondirectional antennaor to all of the unidirectional antennas.

The arrangement of the antennas, receivers, and transmitters at each ofthe ground stations is identical, and the suffixes a, b, and c areemployed to distinguish between identical parts of different stations.While only two directional antennas and receivers are shown at eachstation, it will be understood that three or four such antennas andreceivers may be employed, depending upon the geographical arrangementof the various ground stations with respect to each other.

In accordance with this aspect of the invention, each of the groundstations includes control circuits the details of which are shown inFig. 6, and which will be fully described hereinafter, which controlcircuits are arranged so that upon the receipt of a message at theground station, the receiver or receivers with the weaker signal will bedisconnected or rendered ineffective by the receiver obtaining thestronger signal. Thus, as shown in Fig. 5, receiver 2| 4 at station 20'!will be disconnected and receiver 2I5 will be receiving the signaltransmitted from the aircraft 20I, as indicated by the arrows 202.Likewise at station 209, the receiver 2I5b will be disconnected by thereceiver 2I4b, which is receiving the stronger signal; and at station208 either one of the receivers 2 M11 or 2I5a may take over todisconnect the other, the signals being received at the antennas 2I2aand 2I3a being approximately equal, since the aircraft is operatingdirectly over station 208. The above-described operation of thereceivers takes place before either control energy or audio is appliedto the voice and control channels, and if it is assumed that station 209has the lowest time setting as compared with stations 20'! and 208, thereceiver 2I4b will operate to supply audio and control energy to thesystem, and the receivers and transmitters at stations 20'! and 208 willbe rendered ineiiective, as previously described, due to the highersetting of their time switches. Likewise, as previously described,station 2I0, being outside the transmission range of the aircraft, willhave its transmitter 2I6c turned on to rebroadcast the message receivedat station 209 from the aircraft. If under these conditions abnormalterrain or atmospheric conditions should cause an extension of the transmission range of station 2I0, there will be no closed loop formedbecause the receiver 2I5b at station 209 will be ineffective to supplyaudio or asset-91 51 1.9 control energy tothe system, and because of thedirectional characteristics of antenna 2l2b receiver 2141 will notreceive the signal from station 2l0.

In Fig. 6 a portion of a ground station is shown as having threeunidirectional antennas 2, 218, and 2E9 respectively associated withthree receivers 22! 22!, and 222. It will be understood hat the groundstation of Fig. 6, which represents any one of the stations 201, 208,209, or 2H; of Fig. 5, is identical with Station No. l of Fig. la,except that the three antennas and receivers have been substituted forthe receiver 35 and its antenna in Fig. 1a. In Fig. 6 only so much ofthe control equipment of Fig. 1a. is shown as is necessary for propercorrelation of the three receivers to the remainder of the system. Thusthe source of control energy 34, the time control switch 36, and therelays 53 and 54 of Fig. 1a are shown in Fig. 6 either in whole or inpart.

Respectively associated with the receivers 220, 22l, and 222 areselector units 223, 224, and 225, which function, as heretoforeindicated, selectively to lock out two of the three receivers, dependingupon which of them receives the strongest signal. Selector unit 223, asshown, comprises an electric discharge tube 226 having a triode sectionconsisting of a plate 221, a cathode 228, and a control element 229. Inaddition, the tube 226 includes a diode section consisting of thecathode 228 and an anode 230. As shown, the cathode 228 is connectedthrough a suitable resistor 23! to the negative terminal of a battery232 the positive terminal of which is connected to ground, as indicatedat 233. The plate 227 of the triode section is connected through aresistor 234 to the positive terminal of a battery 235 the negativeterminal of which is likewise connected to ground. The diode section ofthe tube is connected in a circuit including the resistor 23 I and thebattery 232, the anode 236 being connected, as shown, to the operatingWinding of a relay 236, the opposite terminal of the winding beingconnected to a resistor 23! which is connected to ground, as indicatedat 238.

The battery 235. has a greater voltage than the battery 232, and theresistors 231 and 234 are so adjusted that with zero potential on thecontrol element 229, the cathode 228 assumes a positive potential withrespect to ground. Accordingly no current flows in the diode section ofthe tube 225. This condition of zero grid potential corresponds to theno-signal condition of the receiver, 220, the grid being connectedthrough a suitable conductor 239 to the automatic volume control circuitof the receiver. As will be understood by those skilled in the art, theautomatic volume control circuit of a conventional receiver supplies anegative bias potential proportional to, the strength of the signalbeing received. When this bias potential is applied to the controlelement 225, the current flowing inthe triode section of the tubedecreases. in proportion to the biasing potential, causing the cathodeto assumea negative potential, whereupon current. flows through thediode section in a circuit which includes the winding of the relay 236,the resistors 23! and 2,31, and the battery 232. The ohmic resistance ofthe winding of the relay23-6 is small compared, to the resistor 23'',and consequently the potential existing at the junction 240 between thewinding and the resistor is nearly the same as the plate voltage of thediodejsection of the tube.

he relay Z36. shown. ncludes tw s ts f normally open contacts 24! and242 and a set of normally closed contacts 243 for selectivelycontrolling the operation of a p ir of relays 244 and 245 which are@fifiQtive to control the connections of the source of control energy 34and the audiooutput of the receiver 226 to the control energy and voicechannels 2| and 23 (Fig. 1a), as will more fully appear hereinafter. Therelay 244 is provided with two sets of normally closed contacts 246 and241 and with two sets of normally open contacts 248 and 249, and therelay 245 includes a single set of normally open contacts 250.

As shown, the selector units 224 and 225 each includes an identicallyarranged electric discharge tube 226 and corresponding relays 236 and245. In plane of the relay 24 4 of the selector unit 223, however, theunit 224 includes a relay 25! that, in addition to the contacts 246,247, 248, and 245 of relay 244, includes another set of normally closedcontacts 252 and another set of normally open contacts 253. Selectorunit 225 includes, in place of the relay 244 of unit 223, a relay 254that is identical with the relay 25| except that the normally closedcontacts 246 are omitted. Aside from the variation in the number ofcontacts on the relays 24 4, 2 5l, and 254, all of the selector. unitsare identical. The connections between the three selector units 223,224, and 225, as will be explained in detail, are such that the relay 236 at'the selector unit for the receiver that is receiving the strongestsignal, operates to energize the other. relays at that selector unit toconnect the audio-output of the associated receiver to the conductor l3!which extends through all of the selector units and terminates at thecontacts I28 of the relay 54, as previously explained in connection withFig. 1a, and to connect, the control energy source to the conductor H6which likewise extends through all of the selector units and whichleads, by way of the conductor HI, to the time switch 3-6, and by way ofthe conductor. |;36 to the contacts I21 of the relay 54-.

Selective operation of; the units. 223, 224, and 225 in accordance withthe strength of the signal received at the associated receiver isaccomplished by interconnecting, the. junction points 246 of, the threeunits, which interconnection may be traced. from the junction 240 atselector, unit 223 thr ugh a, conductor 255 and the contacts 24.! onrelay 244 to a common conductor or bus barv 256 which extends to theothertwo units. At unit 224 the conductor 256 is connected throughthecontacts 241 of the relay 25 i; and the conductor 255 at that unit tothe junction point 240, and at selector unit 225 the conductor 256 isconnected through the contacts 241 of the relay.254. and a conductor 25?to the junction 240. at this selector unit.

By means of this common connection between the junction points 246, theparticular receiver whichiis receiv'ing thestrong est signal willeffectively, prevent operation of the relays 236 associated with theother, receivers for example, if the strongestsignal isbeing received atreceiver 226, the potentialdeveloped by the diode section of the tube226 of selector unit. 223 will be greater than that. developed inthe'diode sections of the other tubes, and-since the potential developedat selector 2.23 is applied through the common connection to.thejunctions 240 at the other units, thediodesections. at the selector.units. 224 and "ceiving the transmitted signal.

mapper:

:{25 will be biased-ofi and the relays 23s at these units will remaininoperative. When the strongest signal is thus received at receiver226,,and the relay 236 of selector unit 223 operates, the contacts 241and 242 of the relay close to establish energizing circuits for therelays 244 and 245 at the unit 223, and the contacts 243 of relay 236open to disable the energizing circuits for the relays 251 and 254 atselector units 224 and 225. The energizing circuit for the relay 244 atunit 223 extends .from the positive side of a suitable source of energy,indicated by a conventional. plus sign, through the contacts 242, aconductor 258, the energizing winding of the relay 244, a conductor 259,the normally closed contacts 252 on the relay 25l at unit 224, aconductor 266, the normally closed contacts 252 on the relay 254 at unit225, and by way of a conductor 26l 'to the negative side of the sourceofenergy which is indicated by a conventional minus sign. The energizingcircuit for the relay 245 at selector unit 223 extends from the positiveside of the source of energy through a conductor 262, the contacts 24I,a conductor 263, and the energizing winding of the relay 245 to thenegative side of the source of energy. Although conventional plus andminus signs have been used to indicate the source of energy for therelays in the selector units, this is merely for purposes ofsimplification, and it will be understood that any suitable source, suchas the control energy source 34, may be employed.

Immediately upon energization of the relay 244, the contacts 249 closeto connect the audio-output circuit of the receiver 226 to the voicechannel of the system, the circuit extending from the receiver 226v byway of a conductor 264, the con-- tacts 249 of the relay 244, theconductor I3I, and

the contacts I28 of the relay 54, which, as explained in connection withFigs. 1a and 1?), will be energized through the time control switch 36,-if this particular station has the lowest time setting of any of theground stations which are re- Likewise closure of the contacts 248onrelay 244 at selector unit 223 and closure of the contacts 250 on therelay 245 connect the source of control energy 34 to the conductor H6which extends, as previously explained, to the time control switch 36and the contacts I21 of the relay 54. This circuit extends from thecontrol energy source 34 through conductors 265 and 266, the contacts248, a conductor 261, and the contacts 256 to the conductor 1 l6.-Simultaneouslywith closure of these various contacts, the contacts 241on the relay 244 at selector unit 223 open to interrupt the commonconnection between the junction points 246 at the various selectorunits. This is important in order to insure that once a selector unithas taken control, due to reception of the strongest signal, the otherselector units and receivers are locked out for the duration of thatparticular message, Unless this lock-out were provided, it might happenthat afterrebroadcast of the aircraft transmitted message is initiatedat other ground stations, a signal arriving from an adjacent ground.station might be stronger than the aircraft signal and cause one of theother receivers through the discharge tube 226 to produce a potential atthe junctions 246 that would bias-off the diode section of the tube thathad initially taken control. 5 v

It will be observed that whichever of the selector units takes control,operation of the associated contacts 241 in the common connectionisolates the energizing'circuit for the relay 236 at that unit to insurethat this relay will remain energized through the entire duration of themessage. Interruption of the common connection will not preventoperation of the other 236 relays in the event that a stronger signal islater received at other receivers, but such operation of the relays 236at other selector units will not interfere with the original selectorunit because of the manner in which various relays are interconnected.

Assume, for example, that selector unit 223 has taken control and thatthe relay 236 at selector unit 224 thereafter operates. Closure of thecontacts 242 of the relay 235 at unit 224 is ineffective to energize therelay 251, because the energizing circuit which would otherwise becompleted is, as will be more fully explained, interrupted at thecontacts 243 of the relay 236 at unit 223 and at the contacts 246 ofrelay 244. Such later operation of the relay 236 at unit 224 will,however, complete an' energizing circuit for the relay 245 at unit 224,which circuit extends from the posi tive side of the source of controlenergy through the conductors 262 and 268, the contacts 241 at ,unit224, and by way of the conductor 263 at that unit to the energizingwinding of the relay 245. Closure of the contacts 250 on relay 245 atunit 224 is ineffective to complete any circuit because the contacts 248on relay 251, which are in series with contacts 256, remain open.

If it is assumed now that the strongest signal is received at receiver222, then relay 236 at the selector unit 225 will operate to energizethe relays 254 and 245 at that unit, and the relays 236 .at units 223and 224 will be biased-01f, as heretofore explained, the energizingcircuit for the relay 254 at unit 225 extending from the positive sideof the source of energy through the contacts 243 of the relay 236 atstation 223, a conductor 269,

': the contacts 246 on the relay 244, a conductor unit 225 extends fromthe positive side of the" source of energy through the conductors 262and 268 to selector unit 225, then through the contacts 241 at unit 225,the conductor 263 and the winding at the relay 245 to the negative sideof the source. Closure of the contacts 248, 249 and 256 at selector unit225 is effective to connect the receiver 222 to the contacts I28 of therelay 54 and to connect the source of control energy 34 to the contacts-I21 of the relay 54 and to the time control switch 36, the receivercircuit extending from the receiver 222 through the -conductor 264, thecontacts 249, and the conductors 215 and HI, and the control energycircuit extending from the source of control energy 34 through theconductor 265 which, as shown, eX-.

tends through all three selector units and by Way of the contacts 248,the conductor 261, and

the contacts 256 at unit 225 to a conductor 216 which is connected tothe previously mentioned conductor H6 which leads through the conductorsIll and B6 to the time control switch and at receiver 226 so as toefiect operation of the relay 236 at selector unit 223, the previouslydescribed energizing circuit for the relay 254 at station 225 will beinterrupted at the contacts 243 at unit 223. This will be ineffective todeenergize the relay 254 at selector unit 225, however, as the relaywill remain energized through a holding circuit which extends from thepositive side of the source oi energy through the conductors 282 and258, which latter conductor xtends through all three selector units andthrough the contacts 24! on the relay 236 at unit 225, conductors 263and 213, the contacts 253 on relay 254-, a conductor 211, and by Way ofthe en-- ergizing winding of the relay 254 and the conductor 26l to thenegative side of the source of energy. Likewise, operation of the relay236 at unit 224 to open the contacts 243 at that unit will beineffective to de-ene'rgize the relay 254 at unit 225 by reason of thejust described holding circuit, and consequently once the relay 236 atunit 225 has taken control, it will retain control and lockout the otherunits throughout the duration of the message.

Each of the relays 244, 2-5I, and 254' isof the slow-to-release type asindicated diagrammatically by the slugs 278, in order to insure that nomatter which of the receivers 220, 22l, or 222 has taken control, theother receivers will remain locked out at the end of theaircraftinitiated transmission until rebroadcasting from adjacentstations has ceased. Assume, for example, that selector unit 223 hastaken control so that the receiver 220' is supplying the audio andcontrolenergy to the sy-stem, if now the rebroadcast of the message froman adjacent ground station should reach antenna 2i8- and' receiver 22lwith suflicient strength to'cause op erationoi the relays 23B" and 24-5at selector unit 224, no objectionable results would occur because"relay 25 atstation 22-4 will not operate, its energizing circuit beinginterrupted at the contacts 246 of relay 244 and-at contacts 2430fthe're'lay 236 at selector unit 223.

Immediately uponterminationof the aircraft transmitted-message receivedat receiver 220; the

contacts 243 of the relay 2-36 at selector unit 223 will close and-iftherelay 244 should likewise operate immediately to-close its contacts246 an energizin circuit would immediately: beestab-'- lished fortherelay 2'5l extending-from the positive side ofthe source of energythrough the-com 252 on-relay 254, and by way of the'conduc'tor' 26! tothe negative side of the source'of energy; If this energizing circuitwere completed im mediatelyupon cessation of the aircraftinitiatedmessage received at receiver 220, the selectorunit 224-would immediatelybecome effective tosupply control energy to the system and a closedloop,such asthe loop 203' of Fig. 4, would be'es-' ta'blished to lock up thesystem, even though no message was being transmitted. Theslow-to'release-characterof relays 244, 251 and254 prevents such alock-up,however, since the supply of control energy to the system" isinterrupted by the contacts 259on relays 245 immediately" upon cessationof the aircraft initiated message and th contacts 246 on the relays 244,25'lor 254, as'the case may be, remainop'enfor a'sufficient'period oftime thereafter to permit the disabling of all rebroadcasting stationsand the release of-allof th relays 236 before-the' con- 24 were 246reclos to riease the moment of the other receivers.

It will be understood that operation of the selector units 223,- 224,and 225 to select the proper one of the receivers associated therewithtakes place prior to operation of the time control switch, since energyis not applied to the time control switch from the control energy source34 until after the selecting operation has taken place. If the timecontrol switch at the particular station happens to have the lowest timesetting of any station at which the message is being received, the relay54 will then operate to supply the control energy and the audio fromthe" selectedreceiver to the control channel and the voice channel. Inall other respects the operation is as described in connection withFigs. 1a and 1b.

While we have shown particular embodiments of our invention, it will beunderstood, of course, that we do not wish to be limited thereto sincemany modifications may be made, and we therefore contemplate by theappended claims to cover any such modifications as fall within the truespirit and scope of our invention.

Having thus described our invention; what we claim and desire to secureby" Letters Patent is:

1. A system for two-way radio communication between aircraft and" groundover distances in eii'c'ess'of the normal range or transmission andreception at the frequency ofsaid c'ommunica tion, comprising aplurality ofground stations spaced' apart distances greater than saidrange at" ground level but close enough to provide overlapping zones ofground-transmitted signals at the usual aircraft altitudes and to permitsimultaneous reception at aplurality ofground stations ofaircraft-initiated signals transmitted at said altitudes, conductormeans extending between sai'd stations providing a voice channel capableof two-way transmission of audio-frequencies and a control energychannelra normally inoperative radio" receiver, sound reproducer" andradio transmitter at each of said stations, each of said radiotransmitters being operable at a frequency within the aircraft receiver'bandpassacceptance"channel at an assigned frequency, means includingsaid channels and automatically responsive'to the initiation of a"'-message'at 'any' one of said ground stations for causing said soundreproduc'ers and said radio transmitters to become operative" toreproduce and broadcast said message at each of said stations, and meansincluding said channels automatically responsive to'the reception of anaircraft-initiated'me'ssage'jat"any one of said stations for causingtheradio receiver" at said one stationandthe sound reproducers iat allstations to become operative to reproduce said message at all of-saidstationsand for causing only the radio transmitters in stations outsidethe range of said initiating aircraft" to rebroadcast said message.

2; A system'ior two-'wa y raidio"communication between ncraa'anc groundo'vei' distances in excess of'the normal'rangebf transmission andreception at the frequency of said communication, comprising aplurality" of ground stations spaced apart distances greater than saidrange at "groundlevel but close enough'to provide overlapping zones" orground transmitted" signals" at the usual aircraft altitudes and topermit simultaneous reception at a plurality of ground stations ofaircraft signals transmitted at-said altitudes, a conductor voicechannel capable of-two 25* way transmission ofa'udio frequenciesextending between said stations, a conductor control energy channelextending between said stations, a nor-: mally inoperative radioreceiver, sound reproducer and radio transmitter at each of saidstations, each of said radio transmitters being operable at a frequencywithin the aircraft re-' ceiver band pass acceptance channel at anassigned frequency, switching devices at each stag tion automaticallyresponsive to the initiation of a messageat any one of said groundstations to cause the transmission of audio and control energy over saidchannels for efiecting operativeness of the sound reproducers and theradio transmitters to reproduce and broadcast said message at each ofsaid stations, and switching devices automatically responsive to thereception of an aircraft-initiated message at any one of said stationsto cause the transmission of audio and control energy over said channelsfor effecting operativeness of the radio receiver at said onestationo'nly and of the sound'reproducers at'all stations to reproducesaid message at all of said stations and for effecting operativeness ofthe radio transmitters only at stations out side the range of saidinitiating aircraft to rebroadcast said message. I

3. A system for two-way radio communication between aircraft and groundover distances in excess of the normal range of transmission andreception at the frequency of said communicae tion, comprising aplurality of ground stations spaced apart distances greater than saidrange at ground level but close enough to provide overlapping zones ofground-transmitted signals at the usual aircraft altitudes and to permitsimule taneous reception at a plurality of ground stations. of aircraftsignals transmitted at said altitudes, conductor means extendingbetweensaid stations providing a voice channel capable of two-way transmissionof audio-frequencies and a control energy channel, a normallyinoperative radio receiver, sound reproducer and radio transmitter ateach of said stations, each of said radio transmit ters being operableat a frequency within the aircraft receiver band pass acceptance channelat an assigned frequency, means including said channels andautomatically responsive to the initiation of a message at any one ofsaid ground stations for causing said sound reproducers and said radiotransmitters to become operative to repro-' duce and broadcast saidmessage at each of said stations, and means automatically responsive tothe reception of an aircraft-initiated message at adjacent ones of saidstationsand including timing means having diiferent settings at adjacentstations for disabling the receiver at all but one of said adjacentstations f orefiecting operativeness of the sound reproducing means atall stations to reproduce said message at all of said stations and foreffecting operativeness of the: transmitters only at stations outsidethe range of said initiating aircraft to rebroadcast said message.

4. A system for two-way radio communication between aircraft and groundcomprising a plu-. rality of ground stations each of which includes atransmitter and a receiver, said stations being spaced apart distancesgreater than the normal and extending between said stations, conductormeans providing a control energy channel between said stations, soundpickup and sound reproducing means at each of said stations, selectivelyoperable means at each station for connecting the pickup means at thatstation to said voice channel and for simultaneously supplying controlenergyto said control channel, automatic means at each of said stationsresponsive to ener gization of said control channel for connecting saidsound reproducing means and said transmitters to said voice channel andfor turning on said transmitters whereby messages originating at any oneof said stations are reproduced and rebroadcast at each of saidstations, receiver controlled means at each of said stations respon-jsive to reception of a radio signal'by the receiver at that station forconnecting the receiver output to said voice channel and for energizingsaid control channel to operate said automatic means, and means at eachof said stations for preventing operation of the said transmittercontrolling automatic means at that station when said cone trol' channelis energized through said receiver controlled means at the same stationwhereby messages received at any one station are reproduced at all ofsaid stations but are rebroadcast only at stations other than said onestation.

5. A system for two-way radio communication between aircraft and groundcomprising a plurality of ground stations each ,of which includes atransmitter and a receiver, said station's being spaced apart distancesgreater'than the normal effective range of said transmittersat groundlevel but close enough to'provide overlappingsignal zones at the usualaircraft altitudes, a long-l distance voice channel capable of'two-waytrans; mission of audio-frequencies and extending be-.. tween saidstations, conductors extending be--.,'

tween said stations providing a control energ'y channel, a sound pickupdevice and a sound re.-' producing device at each station, selectivelyoper-I able switching devices at each stationfor se-j lectivelyconnecting the associated pickup device? to said voice channel and forsimultaneously energizing said control channel, automatic;- switchingdevices at each station responsive to, energization of said controlchannel for con meeting said sound reproducers and said trans-, mittersto said voice channels and for turning on said transmitters wherebymessages originating at any one of said stations are reproduced. andrebroadcast at each of said stations, auto matic control devices at eachstation responsive to reception of a radio signal by the receiver atthat station for connecting the receiver output, to said voice channeland for energizing said; control channel to operate said automaticsWitch-, ing devices, and a discriminating element in thecontrol channelat each station for preventing operation of the transmitter controllingvauto matic, switching device at that station when said control channelis energized through said.

receiver responsive control device'at the same sta tion, wherebymessages received at any one sta-- tion are reproduced 'at all ofsaid-stations but, are rebroadcast only at stations other than said onestation. i

6. A system for two-way radio communication between aircraft and groundcomprising a plu rality of ground stations each'o'f which includes atransmitting-and-receiving site having a trans mitter and a receiverthereat and at least one operating site having a sound pickup device anda sound reproducer thereat, said stations being a a n:

spaced apart distances greater than the normal range of saidtransmitters at ground level but close enough to overlap at the usualaircraft altitudes, means providing long distance and local voicechannels capable of two-way transmission of audio-frequencies extendingbetween said stations and between said sites at said stations, meansproviding control energy channels extending between said stations andbetween said sites at said stations, means at each operating site forselectively connecting the associated sound pickup device to said voicechannels and simultaneously energizing said control channels, automaticmeans at said sites responsive to energization of said control channelsfor connecting said sound reproducers and said transmitters to saidvoice channels and for turning on said transmitters whereby messagesoriginating at any one of said operating sites ar reproduced at eachoperating site and are rebroadcast at each transmitting-and-receivingsite, means at each transmitting-and-receiving site responsive. toreception of a radio signal by the receiver at that site for connectingthe receiver output to said voice channels and for energizing saidcontrol channels to operate saidautomatic meansat othor sites, and meansat each transmitting-and-receiving site for preventing operation of saidautomatic means at that site when said control channels are energizedthrough said receiver responsive means at the same site, wherebymessages received at any one station are reproduced at all operatingsites but are rebroadcast only at stations other than said one station.

'7. A system for two-way radio communication between aircraft and groundcomprising a plurality of ground stations each of which includes atransmitting-and-receiving site having a transmitter and a receiverthereat and at least one operating site having a sound pickup device anda sound reproducer thereat, said stations being spaced apart distancesgreater than the normal range of said transmitters at ground level butclose enough to overlap at the usual aircraft altitudes, along distancevoice channel capable of two-way transmission of audio-frequenciesextending between said operating sites, local voice channels connectingsaid long distance voice channel and each operating site with itsassociated transmitting-and-receiving site, conductors extending betweensaid stations and between said sites at said stations providing controlenergy channels, switching devices at said operating sites forselectively connecting the associated pickup device to said voicechannels and for simultaneously energizing said control channels, otherswitching devices at each operating site responsive to energization ofsaidcontrol channels for connecting the associated sound reproducer tosaid voice channels wherebymessages originating at any selectedoperating site are reproduced at all of said operating sites, switchingmechanism at each transmitting-and-receiving site respon -sive toenergization of said control channels from any one of said operatingsites for connecting said local voice channels to the audio input ofsaid transmitters and for turning on said transmitters whereby messagesoriginating at any of said operating sites are rebroadcast at, all ofsaid stations, receiver controlled. devices at eachtransmitting-and-receiving site responsive to reception of a radiosignal by said receiver for connecting the audio output circuit of thereceiver to said voice channels and for simultaneously enersizing saidcontrol channels to operate said oth r 28 switching devices atlsaidoperating sites whereby messages received at onetransmitting-and-receiving station are reproduced at all operatingsites, and .circuit elements in said local control channels at eachtransmitting-and-receiving site for preventing energization of saidswitching mechanism through said receiver controlled device at the samesite, said switching devices at other stations operating upon saidenergization of said control channels through said receiver controlleddevice at said one station whereby messages received at said one stationare rebroadcast at other stations only.

8. A system for two-way radio communication between aircraft and groundcomprising a plurality of ground stations each of which includes atransmitter and a receiver, said stations being spaced apart distancesgreater than the normal efiective range of said transmitters at groundlevel but close enough to provide overlapping signal zones at the usualaircraft altitudes and to permit simultaneous reception at a pluralityof stations of a message transmitted from aircraft at said altitudes,meansproviding a long distance voice channel capable of two-waytransmission of audio-frequencies and extending between said stations,conductor means providing a control energy channel between saidstations, sound pickup and sound reproducing means at each of saidstations, selectively operable means at each station for connecting thepick-up means at that station to said voice channel and forsimultaneously supplying control energy to said control channel,automatic means at each of said stations responsive to energization ofsaid control channels for connecting said sound reproducing means andsaid transmitters to said voice channel and for turning on saidtransmitters whereby messages originating at any one of said stationsare reproduced and rebroadcast at each of said stations, receivercontrolled means at each of said stations responsive to reception of aradio signal by the receiver at that station for connecting the receiveroutput to said voice channel and for energizing said control channel tooperate said automatic means, means at each of said stations forPreventing operation of the said transmitter controlling automatic meansat that station when said'control channel is energized through saidreceiver controlled means at the same station whereby messages receivedat any one station are reproduced at all of said stations but arerebroadcast only at stations other than said one station, said receivercontrolled means at each station including timing means providing apredetermined time delay between reception of a signal and energizationof said channels by said receiver controlled means, the time delaydiffering at adjacent stations capable of receiving the same signal toprovide sequential operation, and means at each station eifective uponoperation of said transmitter' controlling means for rendering saidreceiver controlled means at that station inefiective to encrgize saidchannels whereby only the receiving station having the lowest time delayeffects energization of said channels.

9. A system for two-way radio communication between aircraft and groundcomprising a plurality of ground stations each of which includes atransmitter and a receiver, said station-s being spaced apart distancesgreater than the normal effective range of said transmitters at groundlevel but close enough to provide overlapping signal zones at the usualaircraft altitudes, and to permit simultaneous reception at a pluralityof stations of a message transmitted from aircraft 29 at said altitudes,a long distance voice channel capable of two-way transmission of audiofrequencies and extending between said stations, conductors extendingbetween said stations providing a controlenergy channel, a sound pickupdevice and a sound reproducing devic at each station, selectivelyoperable switching devices at each station for selectively connectingthe associated pickup device to said voice channel and forsimultaneously energizing said control channel, automatic switchingdevices at each station res sponsive to energization of said controlchannel for connecting said sound reproducers and said transmitters tosaid voice channel and for turningon said transmitters whereby messagesoriginating at any one of said stations are reproduced and rebroadcastat each of said stations, receiver controlled devices at each stationresponsive to reception of a radio signal by the receiver at thatstation for connecting the receiver output to said voice channel and forenergizing said control channel tooperate said automatic switchindevices, a discriminating element in the control channel at eachlstationfor preventing operation of the transmitter controlling automaticswitching device at that station when said control channel isenergizedlthrough said receiver controlled device at the same station,whereby messages received at any one station are reproduced at all ofsaid stations'Ibut are rebroadcast only at stations other than'said onestation, said receiver controlled device at each station including atime delay switch, said time delay switches having different settings atadjacent stations to provide sequential operation of said receivercontrolled l devices, and contacts operable by said transmittercontrolling automatic switching device at each station to render saidreceiver controlled device at that station ineffective to energize saidchannels, whereby only the receiving station having the lowest timesetting effects energization of said voice and control channels.

10. A system for two-way radio communication between aircraft and groundcomprising a plurality of ground stations each of which includes atransmitter and a receiver, said stations being spaced apart distancesgreater than the normal efiective range of said transmitters at groundlevel but close enough to provide overlapping signal zones at the usualaircraft altitudes and to permit simultaneous reception at a pluralityof stations of a message transmitted from aircraft at said altitudes,means providing a, long distance voice channel capable of two-waytransmission of audio-frequencies and extending between said stations,conductor means providing a control energy channel between saidstations, sound pickup and soundjreproducing means at each of saidstations, selectively operable means at each station for connecting thepickup meansat that station to said voice channel and forsimultaneouslysupplying control energy to said control channel, automatic means ateach of said'stations responsive to energization of said controlchannelfor connecting said sound reproducing means and said transmitters tosaid voice channel and for turning on said transmitters whereby messagesoriginating at any oneof said stations are reproduced andrebroadcast ateach of said stations, receiver controlled means at each of saidstations responsive to reception of a radio signal by the receiver atthat station for connecting the receiver output to said voice channeland for energizing said control channel to operate said automatic means,means at each of said stations for preventing operation of the saidautomatic means controlling said transmitter at that stationwhen saidcontrol channel is energized through said receiver controlled means atthe same station whereby messages received at any one station arereproduced at all of said stations but are rebroadcast only at stationsother than said one station, said receiver controlled means at eachstation including time delay means providing a predetermined time delaybetween reception of a signal and energization of said channels by saidreceiver controlled means, the time delay differing at adjacent stationscapable of receiving the same signal, means at each station effectiveupon operation of the transmitter controlling means at that station forrendering the receiver controlled means at the same station inefiectiveto energize said channels, whereby only the receiving station having theshortest time delay effects energization of said channels, and meansenergizable by said time delay means at the conclusion of its time cyclefor disabling the transmitter at that station whereby a message receivedat a plurality of stations is rebroadcast only at stations outside thereceiving range. v

11. In a system for radio communication between aircraft and ground, aplurality of ground stations each of which includes a transmitter and areceiver, said stations being spaced apart distances greater than thenormal effective range of said transmitters at ground level but closeenough to permit simultaneous reception at a plurality of stations ofmessages transmitted from an aircraft flying at the usual aircraftaltitudes, means extending between said stations providing along-distance voice channel capable of two-way transmission ofaudio-frequencies and a control energy channel, receiver controlledmeans at each of said stations responsive to the reception of signals bythe receiver at that station for connecting the audio output of saidreceiver to said voice channel and for energizing said control channel,transmitter control means at each of said stations responsive toenergization of said control channel for connecting said voice channelto said transmitter and for turnin on said transmitter, said receivercontrolled means at each station including timing means providing apredetermined time delay between reception of a signal and energizationof said channels by said receiver controlled means, said time deladifiering at adjacent stations capable of receiving the same signal, andmeans at each station efiective upon operation of said transmittercontrol means at that station for rendering the receiver controlledmeans at that station ineffective to energize said channels, wherebyonly the receiving station having the shortest'time delay effectsenergization of said channels.

'l2i In a system for radio communication between aircraft and ground,'a'plurality of ground stations each of which includes a transmitter and areceiver, said stations being spaced apart distances greater than thenormal efiectlve range of said transmitters at ground level but closeenough to permit simultaneous reception at a plurality of stations ofmessages transmitted from an aircraft fiying'at the usual aircraftaltitudes, means providing a long-distance voice channel extendingbetween said stations and capable of two-way transmission ofaudio-frequencies;

means providing a control energy channel extending between saidstations, receiver controlled means at each of said stations responsiveto the 75 reception of signals by the receiver at that sta:

